A piezoelectric ceramic represented by zinc zirconate titanate (PZT) has been used for a probe of an ultrasonic diagnostic apparatus as an electroacoustic transducer that transmits/receives an ultrasonic wave. Recently, however, a capacitive micromachined ultrasonic transducer (CMUT) having broader-band characteristics than piezoelectric ceramic is widely noticed, and its research and development are promoted (for example, refer to Patent Literature 1).
The CMUT has structure in which an upper electrode is formed on the upside of a lower electrode formed on a substrate with a cavity between the electrodes and the cavity is covered with an insulating film, and is a piezoelectric transducer making use of electrostatic force generated on a membrane on the upside of the cavity by applying voltage between the upper electrode and the lower electrode and causing potential difference. The transmission of an ultrasonic wave is performed because electrostatic force applied to the membrane varies by temporally varying voltage applied to the upper and lower electrodes and the variation oscillates the membrane, and the reception of an ultrasonic wave is performed by detecting displacement of the membrane as voltage variation or current variation in a state in which fixed voltage is applied between the upper and lower electrodes.
In an ultrasonic diagnostic apparatus, maximum transmission sound pressure is important performance that determines imageable maximum depth. Maximum transmission sound pressure of the CMUT is determined by maximum displacement of the membrane, that is, cavity height. Therefore, to acquire desired maximum transmission sound pressure, it is important to secure the cavity height and it is required to measure the precise cavity height. Further, in an ultrasonic probe, multiple transducer units are normally arrayed in a two-dimensional array, and it is required that these transducer units can generate the same ultrasonic waves at the same applied voltage.